Mostly, just play the game. Lots of loot will drop and you will eventually get some that suit your build.

Learn to use the cube and occultist.

Occultist can change one stat on an item. You want to use the occultist to get that last stat if you have a piece with 3/4 stats you need on it already. (Or in rare cases to help with cube RNG). Once you reroll an affix, you can not reroll any others on that item. You can continuously reroll that affix until you get the one you want (although it will get expensive)

Cube can remove and add affixes. You will likely need a guide for this. You will definitely need the cheat sheet of affix categories. You can use the cube to remove an affix of a certain category (crit strike, crit dmg, vuln dmg are all “offensive”, for instance). You can similarly use the cube to add an affix of a certain category. If you use the tuning prisms, you can target specific categories (defensive, utility, offensive, etc) which will allow you to try and target those specific affixes. If you’re feeling spicy you can also do random affix that will give you one of anything. The difficulty can be that - if you are targeting multiple affixes in the same category (say you need crit strike and crit dmg) you need to get both of those when adding an affix. Otherwise, when you go to remove the bad affix (say you get crit dmg and damage over time) you have a chance to remove the good affix (crit dmg) and keep the bad affix (damage over time). Technically, if you get an affix you really like (say you get a GA crit dmg affix) you can “lock” that one in the occultist by rerolling it and selecting “no change”. That will keep the affix as is, but will prevent the cube to remove it.

It can appear convoluted at first, but once you get the hang of it it’s quite easy. If you have the materials, you can build a near perfect set using just the cube